powerpc: Fix 32-bit SMP build

Commit 69e3cea8d5fd526 ("powerpc/smp: Make start_secondary_resume
available to all CPU variants") introduced start_secondary_resume to
misc_32.S, however it uses a 64-bit instruction which is not valid on
32-bit platforms.  Use 'stw' instead.

Reported-by: Richard Cochran <richardcochran@gmail.com>
Tested-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>